The successful exploration for, delineation of, and eventual mining of ore-bodies, as well as the safe construction of massive structures such as high rise buildings and dam walls, and tunneling for civil works etc. are all dependent on a thorough knowledge of the geology of the ground. A three-dimensional (3D) understanding of the subsurface is developed from surface geological mapping, along with geophysical and geochemical surveys. However, the predictive power of such three-dimensional models is substantially limited without data from reliable and accurate measurements derived directly from the subsurface.
Various drilling techniques (such as diamond drilling) are used to bore deep into the earth with the aim of producing continuous cylinders of rock, termed “core”, which samples are then measured and described in a process known as “core logging”. These data are then processed and analyzed to refine the three-dimensional modeling as well as determine such parameters as ore body size and grade, or the potential for other “hidden” ore deposits/extensions, as well as blasting requirements, rippability of the rock and rock mass behavior characteristics (such as slope stability, fragmentation, flow or creep etc.) when excavating or loading with additional weight and so on.
Core logging entails the measurement and detailed description of a wide range of features—the lengths of lithological and alteration intervals for example, the densities of the different rock types intersected, their uniaxial compressive and tensile strengths, the shear strengths of any contained discontinuities, as well as the attitude of all structures, both planar (bedding, fractures, joints etc.) and linear (fault slickensides, acicular mineral alignment etc.) relative to the core axis. Additional tasks include evaluating the length of missing sections of core due either to natural cavities or mechanical grinding and poor drilling practice and defining the lengths of solid core versus rubble or matrix, or badly fractured and/or friable rock.
Current core logging methods do not achieve optimum results. They are largely manual in that data is handwritten on either standard or customized logging forms. These data are then clerically entered into a computer data base and returned to the geologist for editing, validation and processing. This time consuming process is fraught with transcription, editing and audit errors, to the detriment of both the drilling program and the ground interpretation/modeling exercise.
Three-dimensional ground modeling is further severely hampered by the fact that the collection of subsurface structural data is still erroneously considered too specialized and expensive to run routinely. In reality only two procedures to prepare the core for such measurements are needed. Firstly, the intersection of the in-situ geographic vertical plane with the rock core has to be marked along the surface of the cylinder. Several different instruments can be used to indicate the position of such a reference line, which when drawn on the core is known as an “orientation line”. Then the attitude, or inclination, of the core axis has to be surveyed along the entire length of the bore, which data together with measurements of the down-hole change in core axis direction, or azimuth, is known as a “borehole survey”. Given this information the angular attitude of planar or linear structures found in the core can be measured relative to the core axis and orientation line, and the results computed to derive the actual in-situ dip and dip direction, or plunge and trend of the planar and linear geological structures respectively.
The borehole survey is also required in order to calculate the geographic coordinates of features logged in the core, before the data can be processed to construct a meaningful three-dimensional model. Such modeling then requires statistically valid data to develop a reliable schematic of the ground. One of the main tools for statistically analyzing three-dimensional geological data is the stereographic projection, or Stereonet, on which the geologist should plot each and every structural data reading. The resulting Stereoplot is then contoured from which statistically valid mean values are derived for the various planar and linear structures logged in the core.
This systematic processing approach enables filters to be applied to the data at any stage of the logging process not only to evaluate accuracy, but also to display how the structures (faults, joints, shears etc.) change with depth in the ground as well as how they are influenced by the different lithologies, rock properties etc.